The drilling of oil or gas wells commonly involves the use of an apparatus made up of a drill string having a steering tool or a plurality of logging tools positioned within or affixed to the drill string near its lower end. A communication wireline cable is often used for transmitting to the surface the information gathered by such a tool or tools.
Devices known as "mud motors" or "turbodrills" are often employed in an operation for changing the direction of a wellbore during drilling operations. These devices may be attached near the lower end of the drill string above the drill bit. During mud motor drilling operations, high pressure drilling mud is circulated down the interior of the drill string, through the mud motor and drill bit and up the annulus of the wellbore. The action of the mud on turbine blades built into the mud motor rotates the drill bit without rotating the drill string. During such mud motor drilling operations, a steering tool is often positioned inside the drill string above the mud motor. The steering tool monitors the inclination and azimuth of the wellbore during drilling, so that course corrections may continuously be made.
One common method for establishing communication with a steering or other downhole tool involves insertion of a wireline cable through a port in an element of drill string known as a "side-entry sub", to form a hardwire link between the downhole tool and the surface. Thus, a fixed length of such a cable is contained inside the drill string below the side-entry sub. The cable between the side-entry sub and the surface extends along the outside of the drill string.
One benefit of the wireline cable's side-entry through a side-entry sub stems from the fact that new drill pipe sections may be added to the drill string without first withdrawing the cable from the wellbore. In contrast, if the wireline cable is run totally inside the drill string from the surface to the downhole tool, the most feasible method for adding a pipe section to the drill string involves the complete removal of the cable from the drill string, followed by addition of the new pipe section, and re-installation of the cable. Inclusion of a side-entry sub in the drill string renders unnecessary such repeated cable re-installation, increasing the speed of the operation, and lowering its cost.
Various schemes have been proposed for sealing a cable at its point of entry through the side-entry sub. For example, U.S. Pat. No. 4,062,551 to Base and U.S. Pat. No. 4,200,297 to Tricon disclose apparatus for permitting the sealed side-entry of a cable into the interior of a drill string. Base discloses a cylindrical cable seal unit which may be fitted between adjacent pipe sections in a drill string to form a substantially continuous assembly. A port is bored through the side of the cable seal unit for passing a cable therethrough. The interior end of the port is of smaller diameter than the outer end, thus defining a shoulder. An annular packing seat is positioned in the shoulder. Packing material is compressed against the packing seat and the cable is positioned through the port to seal the port around the cable. Tricon discloses a similar apparatus for permitting the entry of a cable through a port in a housing attached to the side of a section of drill pipe by providing a seal between the cable and the housing when the cable is positioned in the port.
However, neither Base nor Tricon discloses apparatus which automatically seals the port on removal of the cable. Thus, either apparatus would permit fluid flow through the port between the interior and exterior of the drill string in the event that a cable was removed.
It may be desirable to remove the cable from the drill string during drilling or logging operations, for example, if the cable becomes damaged. Another important reason for removing the cable would be to vacate the drill pipe after it has become stuck. Retaining a length of cable inside such stuck drill pipe would severely hamper the running of tools, such as free-point indicators and explosive devices, down the interior of the drill string. Also, if the drill string must be rotated while the side-entry sub section is positioned below the surface, that portion of the cable extending along the outside of the drill string and running to the surface will probably sustain damage and could severely interfere with drill string rotation. When it is desired to remove the cable from the drill string, the cable usually is removed by retrieving it through the port in the side-entry sub section. In so removing the cable from the port, however, if the cable port is left open and drilling fluid or "mud" is being circulated, the drilling mud will short circuit the normal drilling mud flow path and instead will flow through the open cable port of the side-entry sub. The prior art has not addressed this problem. The present invention provides a solution to this problem by automatically closing the side-entry port on removal of the cable therefrom.